Exit the async scheduler after running the tests.
(>>) operator for lwt implementation.
Pull out Cohttp_async.IO into a separate file.
Merge branch 'master' of github.com:yallop/ocaml-cohttp

Conflicts:
	async/cohttp_async.ml
Bring lwt implementation to a working state; refunctorize a bit.
Share signatures.
Bring lwt/mirage implementation back to a well-typed state.